1. Field of the Invention
The present invention relates to a system for reducing shift shock upon shifting of an automatic power transmission associated with transition from coasting state with an accelerator pedal released to power-on driving state with the accelerator pedal depressed.
2. Description of the Related Art
A V-belt type or toroidal-type continuously variable transmission is constructed to determine a optimal power transmission ratio (speed ratio) at respective vehicular driving condition on the basis of a load condition of an engine (normally, throttle valve opening) and a vehicle speed at the current speed ratio of the transmission, and performs shifting toward the thus determined optimal transmission speed ratio.
Accordingly, under power-on transition from a coating state where an accelerator pedal is released to a power-on state where the accelerator pedal is depressed, the continuously variable transmission performs shifting of speed ratio (normally down shifting) in response to re-depression of the accelerator pedal.
Here, the magnitude of shift shock associated with shifting of the speed ratio is significantly affected as explained below in connection with an actual shifting initiation timing and a rising timing of the engine output in response to the accelerator pedal operation. Namely, when shifting of the automatic transmission has relatively high response in comparison with rising of the engine output, as can be appreciated from the output torque waveform of the transmission as shown by "a" in FIG. 5, since a torque converter is normally placed in lock-up state, dropping of torque is initially caused in response to shifting, and jerking shock is subsequently caused due to the rising of the engine output. Conversely, when the response of shifting in the power transmission is lower in comparison with the rising of the engine output, as can be appreciated from the output torque waveform shown by "b" in FIG. 5, since the torque converter is in the lock-up state, jerking shock is caused due to an abrupt rising of the engine output.
These problems of shift shock become significant since the continuously variable transmission permits an expansion of the lock-up range of torque converter in view of the transmission characteristics thereof, and employs the expanded lock-up range, and the engine in front of the transmission is normally equipped with a fuel cut-off system, which terminates fuel supply to the engine for improving the fuel economy during coasting of the vehicle.
On the other hand, when the rising of the engine output is synchronized with shifting of the automatic transmission, as can be appreciated from the waveform of the output torque of the transmission shown by "c" in FIG. 5, shift shock becomes small even at the lock-up condition of the torque converter since the drop of the torque due to shifting and rising of the engine output are canceled.
Discussion will be given for the fact that the shift shock becomes small when the rising of the engine output and the down shifting of the automatic power transmission is synchronized. FIG. 6(a) shows a chronographical variation of acceleration of vehicular longitudinal vibration in the case where the engine output torque is increased stepwise at the moment t.sub.1 with maintaining the speed ratio of the continuously variable transmission constant. It can be clearly appreciated that the acceleration of the vehicular longitudinal vibration due to increasing of the engine output torque rises at initiation timing of increasing the engine output torque. On the other hand, as can be appreciated from FIG. 6(b), which also shows the chronographical variation of the acceleration of the vehicular longitudinal vibration, when the speed ratio of the continuously variable transmission is increased by down shifting at the moment t.sub.1 with maintaining the engine output torque constant, the acceleration of the vehicular longitudinal vibration is lowered from the initiation timing of increasing the speed ratio. Accordingly, variation characteristics of the acceleration of the vehicular longitudinal vibration in the former case of increasing the engine output torque and in the latter case of increasing the power transmission ratio change, the phase thereof becoming opposite in phase as can be appreciated from comparison during the period through timing of t.sub.1, t.sub.2 and t.sub.3.
Here, upon simultaneous occurrence of both phenomena in such a manner that they are not synchronized with each other, the composite acceleration of the vehicular longitudinal vibration becomes amplified to become greater, as shown by solid line in FIG. 7, to cause a large shift shock. In contrast, when both phenomena are synchronized, the composite acceleration of the vehicular longitudinal vibration becomes canceled for phase cancellation effect, as shown by the solid line in FIG. 8, to reduce the shift shock.
However, in practice, the conventional continuously variable transmission does not perform a control for establishing synchronization between rising of the engine output and shifting of the transmission. Moreover, it is quite unlikely that synchronization between rising of the engine output and shifting of the transmission is established, since the rising of the engine output associated with fuel recovery in response to termination of operation of the fuel cut-off system and shifting of the continuously variable transmission are caused with mutually independent lags in response. Particularly, the lag in response of shifting of the continuously variable transmission is much larger than the lag in response of rising of the engine output. Furthermore, the response lag in the shifting of the continuously variable transmission may be fluctuated depending upon the temperature of the working fluid and so forth. Accordingly, the conventional continuously variable transmission may cause a large shift shock at the transition from a coasting state to a power-on driving.